1. Field of the Invention
The present invention relates to a valve apparatus for splitting the flow path of fluid by a pair of plugs provided to one shaft thereof.
2. Description of the Prior Art
FIG. 38 is a sectional view showing a conventional valve apparatus having a double-sheeted structure, and in this figure, a reference numeral 1 denotes an upper cover of the valve apparatus; 2 illustrates a lower cover; and 3 shows a valve case (a valve main body). A reference numeral 5 represents an upper plug, and 6 indicates a lower plug. The upper plug 5 and the lower plug 6 are provided on a valve shaft 4 so as to make the same axis, and are shifted in the upper and lower directions along with the valve shaft 4. A reference numeral 7 denotes an upper sheet ring making a pair with the upper plug 5, and 8 illustrates a lower sheet ring making a pair with the lower plug 6. A reference numeral 9 shows an inflow opening of the valve case 3, and 10 represents an outflow opening of the valve case 3. A reference numeral 11 indicates an upper port section defined by the upper sheet ring 7, and 12 denotes a lower port section defined by the lower sheet ring 8. A reference numeral 11a illustrates an upper current path in which fluid from the upper port section 11 flows, and 12a shows a lower current path in which fluid from the lower port section 12 flows. A reference numeral 2a represents a guide hole provided to the lower cover to guide the lower plug 6.
Next, an operation will be described.
The fluid flows into the valve case 3 from the inflow opening 9, then flows out from the outflow opening 10. In the case that the amount of lift is 0%, i.e., the upper plug 5 and the upper sheet ring 7 closely touch and the lower plug 6 and the lower sheet ring 8 closely touch each other, there is no inflow and outflow of the fluid.
When the upper plug 5 and the lower plug 6 are shifted by the valve shaft 4, the fluid from the inflow opening 9 flows in the upper current path 11a via the upper port section 11, and also flows in the lower current path 12a via the lower port section 12. The fluid passed in the upper current path 11a and the lower current path 12a flows out from the outflow opening 10.
Since the conventional valve apparatus is constituted as described above, there are problems as follows.
a) In the conventional valve apparatus, two pairs of plug and sheet ring are provided so as to split the fluid flowing into the upper and lower portions, and it is characterized in that fluid power unbalance between such upper and lower flows is mostly offset. However, as will be understood from FIG. 39 showing an example of a relation between the amount of lift and the unbalance power, although the unbalance power is urged in a positive direction (a direction to open the valve) in the case that the amount of lift is 0 to 50%, the unbalance power is suddenly changed to the direction to a negative direction (a direction to close the valve) after the amount of lift is over 50%, then increases up to 100%.
Although the change of the unbalance power from the positive direction into the negative direction is at the point of 50% of the amount of lift in FIG. 39, if considered with other examples, the change of the unbalance power from the positive direction into the negative direction is caused in a range of 30 to 50%.
While the unbalance power is urged to the positive direction, an operational device (not shown) controls the valve in the direction to close, with the upper plug 5 and the lower plug 6 being pushed through the valve shaft 4. In this state, if the unbalance power is changed into the negative direction, since the upper plug 5 and the lower plug 6 operate so as to close the valve, the operational device controls the valve in the direction to open for keeping the degree of opening of the valve. Besides, the more the change of the unbalance power in the negative direction is rapid, the more the control of the operational device becomes rapid, thereby causing a problem that the control of the operational device becomes unstable when the amount of lift is over a specific range.
b) The guide hole 2a is formed in the lower cover 2, and a lower end portion of the lower plug 6 is inserted into this guide hole 2a to carry out the guiding for the lower plug 6. Because of this, the depth of the guide hole 2a is required to accommodate at least the length of the lower portion of the lower plug 6, thereby causing a problem that the valve apparatus in itself becomes long in the lower side direction.
c) There is a problem that enough space margin under the bottom surface of the lower cover 2 is required for opening the lower cover 2 at the maintenance for the main body of the valve apparatus.
d) There is a problem that operability is bad if there is not enough space at the lower portion of the main body of the valve apparatus, when the lower cover 2 is removed to perform the maintenance of the valve apparatus itself, after the valve apparatus is connected with pipes.
e) Because the upper plug 5 and the lower plug 6 have approximately the same shape, a flow ratio between an upper fluid and a lower fluid at the time of synchronous valve opening becomes 1:1, and if the flow ratio is thus the same, a thrust (unbalance power) force to each of the upper and lower plugs 5 and 6 in the axis direction as a fluid reaction force is rapidly changed near the degree of half opening in which the upper and lower plugs 5 and 6 have actual opening degree, thereby causing a problem that the valve control becomes unstable or uncontrollable.
In other words, the thrust in the axis direction caused by the difference of fluid pressures operated thereto is changed largely near the half opening degree, and is rapidly turned over, in the both upper and lower plugs 5 and 6 which are formed as approximately the same shape such that the flow ratio between the upper fluid and the lower fluid at the valve opening becomes equal to each other. Such rapid reverse of the thrust caused by generating an unstable drawing flow of the fluid around the lower plug 6 having approximately the same shape as the upper plug 5 as shown in FIG. 40(a), thereby to generate a portion P in which the thrust is largely changed, even if the plug position is constant as shown in FIG. 40(b).
Here, if the plug opening is controlled by a diaphragm operational device for adjusting the amount of the fluid at the time when the thrust is turned over, a panting action occurs, therefore, there is a problem that the control by the diaphragm operational device becomes unstable, or becomes uncontrollable with the diaphragm operational device being made incongruous.
f) The fluid flowing out from a gap generated between the upper plug 5 and the upper sheet ring 7 flows into the upper current path 11a, and the flowing direction of the fluid is changed into the direction of the outflow opening 10 of the fluid at the upper current path 11a, after colliding with the bottom surface of the upper cover forming an upper inside surface of the upper current path 11a, thereby causing a problem that noise or oscillation is generated with the fluid flowing being tumbled at this time.
g) The fluid flowing out from a gap generated between the lower plug 6 and the lower sheet ring 8 flows into the lower current path 12a, and the flowing direction of the fluid is changed into the direction of the outflow opening of the fluid at the lower current path 12a, after colliding with the upper surface of the lower cover 2 forming a bottom surface of the lower current path 12a, thereby causing a problem that noise or oscillation is generated with the fluid flowing being tumbled at this time also.
h) In adjustment of the sheet pitch in the vertical direction of the upper sheet ring 7 and the lower sheet ring 8 with reference to the valve case 3, the upper sheet ring 7 and the lower sheet ring 8 are screwed in the valve case 3 respectively, and are removed after the sheet surface at such a screwed state is measured, then the sheet surface is further processed, as well as screwed again into the valve case 3 to measure, thus it is repeated. Owing to this, in the valve apparatus having a plurality of plugs, it was a complicated operation.
i) Since the upper sheet ring 7 and the lower sheet ring 8 are fixed to the valve case 3 respectively by screws, adjustment of the upper sheet ring 7 is carried out after the upper cover 1 is removed from the valve case 3, and adjustment of the lower sheet ring 8 is performed after removing the lower cover 2 from the valve case 3, thereby causing a problem that the operation of the adjustment takes time.
j) Since the guide hole 2a having a predetermined depth must be provided to the lower cover 2, and this causes the thickness of the lower cover to become large, so that the amount of the projection of the bottom of the valve case 3 becomes large, there is a problem that the lower cover 2 cannot be removed because enough lower operational space cannot be obtained after the valve apparatus is installed.
k) In addition, a plug guide portion 13 is formed as one body with the plugs 5 and 6, so that if the plug guide portion 13 is damaged by oscillation or erosion, or is required to change on a maintenance, the plugs 5 and 6 must be also changed, thereby causing a problem of making the valve apparatus uneconomical.